Platelet size is a determinant of platelet function. As platelet size in diabetics has been reported to be larger than normals, we have measured mean platelet volume (MPV) in diabetics simultaneously with fasting plasma glucose, fructosamine, and hemoglobin A1. MPV was positively correlated with fasting plasma glucose and fructosamine. In a sequential observations for a few months of 22 patients, all of whom showed decreasing fasting plasma glucose, MPV decreased in 10 patients, however, increased in only 2 patients. These findings suggest that MPV would be affected with diabetic control.